1. Field of the Invention
The present invention relates to a sintered oil retaining bearing and a fabrication method thereof. More particularly, the present invention relates to a sintered oil retaining bearing having a bearing bore formed at a bearing main unit made of a porous sintered alloy to insert a rotary shaft, and a method of fabricating such a sintered oil retaining bearing.
2. Description of the Background Art
A sintered oil retaining bearing formed of a porous sintered alloy and used infiltrated with a lubricant is widely employed as the bearing of a rotary shaft for various apparatuses since it can be used for a long period of time without having to supply oil.
In this type of sintered oil retaining bearing, a rotary shaft is inserted through the bearing bore of the bearing main unit formed of a porous sintered alloy. By the pumping action corresponding to rotation of the rotary shaft, the lubricant output from a plurality of small pores (open pore) infiltrated with oil in the bearing main unit and the exuding lubricant due to dilation caused by frictional heat form a lubricating film at the sliding surface of the bearing bore. The sliding surface between the rotary shaft and the bearing main unit is lubricated by this lubricating film.
In such a sintered oil retaining bearing, many open pores are formed to infiltrate the sliding surface of the bearing bore with lubricant. If the rotation stops at a very low temperature environment, the lubricant will be taken up into the open pores at the closing pore portion (open pore size decreased). This means that there will be no lubricant at the sliding surface between the rotary shaft and the bearing main unit when rotation starts, resulting in local contact between the rotary shaft and the bearing main unit to cause noise.
An object of the present invention is to provide a sintered oil retaining bearing that can have suction of the lubricant at the closing pore portion prevented and that can have sufficient lubricant supplied at the closing pore portion, and a fabrication method thereof.
According to a sintered oil retaining bearing of the present invention, a bearing bore through which a rotary shaft is to be inserted is formed in a bearing main unit made of a porous sintered alloy. The inner circumferential wall plane which becomes the bearing bore includes a closing pore portion where air permeability is not more than 0.3xc3x9710xe2x88x923 darcy.
Since the air permeability is as low as not more than 0.3xc3x9710xe2x88x923 darcy in the sintered oil retaining bearing of the present invention, suction of the lubricant into the open pore at the closing pore portion can be suppressed even if rotation is ceased under a very low temperature environment. Since there is sufficient lubricant at the sliding surface between the rotary shaft and the bearing main unit when rotation starts, the problem of local contact between the rotary shaft and the bearing main unit to cause noise can be prevented.
In the above sintered oil retaining bearing, the air permeability of the closing pore portion is preferably not more than 0.1xc3x9710xe2x88x923 darcy.
Accordingly, suction of lubricant into the open pore at the closing pore portion can be further suppressed.
In the above sintered oil retaining bearing, the inner circumferential wall that becomes the bearing bore preferably includes a center inner circumferential wall located at the center portion of the inner circumferential wall, one end side inner circumferential wall and other end side inner circumferential wall located at one end side and the other end side, respectively, of the center inner circumferential wall in the direction of the rotary shaft. The center inner circumferential wall corresponds to the closing pore portion, and has an air permeability smaller than that of the one end side inner circumferential wall and the other end side inner circumferential wall.
Accordingly, oil can be supplied sufficiently at the closing pore portion from the one end side or other end side of the inner circumferential wall that is porous.
In the above sintered oil retaining bearing, the effective porosity of the bearing main unit is at least 20% by volume.
Accordingly, oil can exude from the open pores at the sliding plane except for the region of the closing pore portion. By supplying the exuding oil to the closing pore portion, a secure lubricating film can be formed at the closing pore portion.
In the above sintered oil retaining bearing, the bearing main unit is preferably formed of at least one type of material selected from the group consisting of Fe, Fexe2x80x94Cu system and Cuxe2x80x94Sn system.
By selecting such materials, an economical sintered oil retaining bearing of high hardness and favorable wear resistance can be provided.
In the above sintered oil retaining bearing, the bearing main unit preferably is formed of a material of the Fexe2x80x94Cu system, and includes Fe reduced powder.
By this Fe reduced powder, air permeability not more than 0.1xc3x9710xe2x88x923 darcy can be achieved more easily than with atomized powder. Although the usage of atomized powder may be more preferable if simply the density is to be improved, the density at portions other than at the proximity of the surface will become too high from the standpoint of storing oil. The mold lifetime corresponding to the closing pore process will become shorter. In contrast, the usage of Fe reduced powder allows low density and high air permeability. Furthermore, closing pore can be performed more easily than with atomized powder, and the lifetime of the mold can be increased.
In the above sintered oil retaining bearing, the Fe reduced powder is preferably included at least 45% by mass and not more than 60% by mass of the entire mass.
If the amount of Fe reduced powder is less than 45%, the amount of Cu is so large that the material cost will become too high. If the amount of Fe reduced powder exceeds 60%, the bearing main unit will become so hard that the closing pore process will become difficult, and low air permeability cannot be achieved.
In the above sintered oil retaining bearing, the one end side inner circumferential wall has a first tapered portion where the hole diameter becomes larger as a function of approaching the one end side. The other end side inner circumferential wall has a first straight portion extending in the direction of the rotary shaft while maintaining the bore diameter of the bearing bore. The center inner circumferential wall includes a second straight portion extending in the direction of the rotary shaft while maintaining the diameter of the bearing bore at the other end side, and a second tapered portion at the one end side, having a larger bore diameter as a function of approaching the one end side, and an inclination angle smaller than that of the first tapered portion.
The tapered portion at the one end side of the bearing bore allows oil to be stored at the region between the tapered portion and the rotary shaft. The stored oil can be sequentially supplied to the sliding surface. This tapered portion can be formed using a mold that has a tapered portion at the time of compacting the powder. Since the mold can be formed in a tapered configuration, breakage of the mold at the time of compacting the powder can be prevented.
The tapered portion at the one end side inner circumferential wall and the straight portion of the other end side inner circumferential wall contribute to the storage and circulation of oil in ordinary operation. The tapered portion at the closing pore portion of the center inner circumferential wall contributes to storage of oil during ordinary operation, and the straight portion of the closing pore portion contributes to the storage of the lubricant at the sliding surface and circulation of oil until the operation starts in a very low temperature environment.
According to another aspect of the present invention, a fabrication method is provided of a sintered oil retaining bearing having a bearing bore formed to insert a rotary shaft in the bearing main unit made of a porous sintered alloy. The fabrication method includes the steps set forth in the following.
First, by powder compacting, a compact of the bearing main unit is formed so as to have an excessive portion in the bearing bore. A sintered compact is obtained by sintering the compact. By sizing the compact, the excessive portion is compressed by the mold, whereby a closing pore portion of air permeability lower than that of other portions is formed in the bearing bore.
By applying a compression process with the mold during the sizing process to form a closing pore portion according to the fabrication method of a sintered oil retaining bearing of the present invention, air permeability not more than 0.3xc3x9710xe2x88x923 darcy can now be achieved that was not possible by the conventional method.
In the above fabrication method of a sintered oil retaining bearing, the inner circumferential wall that becomes the bearing bore includes a center inner circumferential wall located at the center region of the inner circumferential wall, and one end side inner circumferential wall and other end side inner circumferential wall located at one end side and the other end side, respectively, of the center inner circumferential wall in the direction of the rotary shaft. The excessive portion is formed at the center inner circumferential wall.
Accordingly, sufficient oil can be supplied from either the one end side inner circumferential wall or the other end side inner circumferential wall that is porous.
In the above fabrication method of a sintered oil retaining bearing, the step of forming a compact preferably includes the step of forming an excessive portion so as to include a tapered portion that has a larger bore diameter of the bearing bore as a function of approaching the one end side inner circumferential wall or the other end side inner circumferential wall from the excessive portion, and an r portion (radius portion) having a curvature (curved portion), located between the tapered portion and the one end side or other end side inner circumferential wall.
By the tapered portion and radius portion, an abrupt stepped portion between the excessive portion and the one end side or other end side inner circumferential wall can be eliminated. Therefore, generation of a recessed portion caused by the stepped portion being pulled by the core rod during the sizing process can be prevented.
In the above fabrication method of a sintered oil retaining bearing, the dimension in diameter of the excessive portion in the diametral direction is at least 4% and not more than 10% the inner diameter of the bearing bore after the sizing process.
Accordingly, the desired air permeability can be achieved.
In the above fabrication method of a sintered oil retaining bearing, the dimension of the excessive portion in the diametral direction is at least 0.35 mm and not more than 0.9 mm in diameter.
Accordingly, the desired air permeability can be achieved.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.